[anjuta-list] How to define the entry?

["????????" <736036594 qq com>]

Dear Sir:
    I want to write a GUI with python. There has existed a entry.

The following is my code:
----------------------------------------------------------------------------------------------
from gi.repository import Gtk, GdkPixbuf, Gdk
import os, sys, serial

ser =serial.Serial('/dev/ttyUSB0',9600,parity=serial.PARITY_ODD)

class GUI:
    def __init__(self):

        self.builder = Gtk.Builder()
        self.builder.add_from_file(UI_FILE)
        self.builder.connect_signals(self)

        window = self.builder.get_object('window')

        window.show_all()
    def on_button_clicked (self, button):
        text =entry.get_text()                                    #I want to get the text of entry and sent it to serial port
        ser.write(text)
        
    def destroy(window, self):
        Gtk.main_quit()

def main():
    app = GUI()
    Gtk.main()
        
if __name__ == "__main__":
    sys.exit(main())

-------------------------------------------------------------------------------------------

But when I execute it, it returns this error:

NameError: global name 'entry' is not defined.

Could your tell me how to define a entry and Where I should put the statement on?